The GTZ results I've seen are 2-3 degrees lower temps at load (edit: vs. the GT). Little difference at idle. It's a sweet looking block on the board, that's for sure.
I'm running the Primochill PC Ice right now, but just picked up some Fluid XP+ to try out.

The 655 pump is still king of the hill, as far as I know, if you're running 1/2".